This invention relates generally to angle headed blind fasteners, and more particularly concerns the construction of such fasteners to overcome problems and defects stemming from fastener head and shank reception in metallic and non-metallic work bores and counterbores.
In the past, the use of angle headed fasteners to connect work panels, as for example aluminum, titanium or composite panels as employed in aircraft structure, has given rise to numerous problems. Among these were the practice of providing countersinks in the work slightly oversize in relation to the outer edges of the received fastener heads. The required removal of such material to produce the oversize countersinks resulted in undesirable notch sensitivity reduced shear bearing area, and decline of fatigue life of the structure. In this regard, most fasteners are designed to withstand shear loading, and in thin sheet applications the shear bearing area is reduced, magnifying the loading per square inch on the bearing area, thereby inducing sooner failure. Knife edge conditions associated with the countersink contribute to the problem. Also corrosion problems developed; for example, protective material filled into annular recesses about the heads tended to work loose during flexing of aircraft structures, so that corrosive fluids could gain access to the work bore along the fastener length, weakening the structure by producing corrosion and stress corrosion. Also, since the fastener head periphery does not completely engage the work bore due to the oversize relation, the development of desirable radial compression and peripheral tension in and within the elastic limits of the work material about the fastener head was not possible.
In addition, vibration induced loosening of blind fasteners connecting aircraft skins has been a very serious problem, leading to fatigue and shear failures, and corrosion problems.